Ultracapacitors are energy storage devices. They are sometimes referred to as "double layer capacitors", and are notable for their ability to store more energy per unit weight and volume than conventional capacitors. They are also able to deliver the stored energy at a higher power level than is possible with other electrochemical devices, such as batteries. In general, an ultracapacitor cell contains two porous electrodes that are isolated from electrical contact by one or more porous separators or physical separating mechanism. An electrolytic solution usually surrounds and impregnates the electrodes and separation zone. By this means, ionic current can flow between the electrodes, and electrical power is produced by the ultracapacitor. The current is generated at the electrodes, and is usually conducted out of the cell by a current collector, which is in contact with each electrode. Thus, an ultracapacitor cell is usually composed of two electrodes contacting current collectors; an electrolyte; a separator; and some method of delineating the boundary of a cell, such as a sealed container. Frequently, multiple ultracapacitor cells are arranged in series connection, often referred to as "stacks", to yield an increased operating voltage. Furthermore, ultracapacitor cells or series stacks may be connected in parallel arrangement to yield increased power capability. These types of ultracapacitors can be found in a wide variety of devices, such as memory protection circuits, cellular phones, toys, and other devices needing high power for a relatively short duration.
In operation, an electrode potential, or voltage, exists across the electrodes in the ultracapacitor. Ionic current can then flow as anions are discharged from the surface of the positive electrode where they were stored. In similar fashion, cations are discharged from the surface of the negative electrode. The electrical charges are held on the surface of the porous electrode, whereupon they generate an electrode potential. This potential increases in a generally linear fashion with the quantity of charge species, or ions, being stored on the surface.
The performance of an ultracapacitor is affected by a variety of factors. One important characteristic which relates to the operating performance of the ultracapacitor is its internal resistance, which is usually designed to be as low as possible. In certain types of ultracapacitors, the presence of oxygen and other contaminants therein undesirably increases the resistance, and may have other deleterious effects on the operation and life of the ultracapacitor. Residual gasses can also initiate corrosion within the ultracapacitor, which could in turn damage the device. Moreover, these gasses could cause the ultracapacitor to rupture, for example, when external pressure is applied to sealed cells. (External pressure is commonly employed to reduce the internal resistance of ultracapacitor cells).
Clearly, the presence of ambient gasses (those gasses in the environment in which the device is being manufactured) is related directly to the manufacturing process used to make the ultracapacitor, and specifically to the final process steps, i.e., the sealing of the ultracapacitor package. Thus, one can readily understand that new methods for sealing an ultracapacitor would be welcome in the art. The methods should ensure that the ultracapacitor maintains the necessary attributes, such as low internal resistance and an acceptable operating voltage. Moreover, the methods should not be unduly complicated, or involve excessive expense. Furthermore, the resulting ultracapacitors should be capable of withstanding high external pressures without rupture of the cell containment, and without being otherwise damaged.